1. Field of the Invention
The invention relates to the manufacture of elements faced with superhard material, and particularly to preform elements comprising a facing table of superhard material having a front face, a peripheral surface, and a rear surface bonded to a substrate of material which is less hard than the superhard material.
2. Description of Related Art
Preform elements of superhard material are often used as cutting elements on rotary drag-type drill bits, and the present invention will be particularly described in relation to such use. However, the invention is not restricted to cutting elements for this particular use, and may relate to preform elements for other purposes. For example, elements faced with superhard material, of the kind referred to, may also be employed in workpiece-shaping tools, high pressure nozzles, wire-drawing dies, bearings and other parts subject to sliding wear, as well as elements subject to percussive loads as may be the case in tappets, cams, cam-followers, and similar devices in which a surface of high wear resistance is required.
Preform elements used as cutting elements in rotary drill bits usually have a facing table of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. This forming process is well known and will not be described in detail.
Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier. Alternatively, the substrate itself may be of sufficient thickness as to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a socket in the bit body, without being brazed to a carrier. The bit body itself may be machined from metal, usually steel, or may be molded using a powder metallurgy process.
Such cutting elements are subjected to extremes of temperature during formation and mounting on the bit body, and are also subjected to high temperatures and heavy loads when the drill is in use down a borehole. It is found that as a result of such conditions spalling and delamination of the superhard facing table can occur, that is to say the separation and loss of the diamond or other superhard material over the cutting surface of the table. This may also occur in preform elements used for other purposes, and particularly where the elements are subjected to repetitive percussive loads, as in tappets and cam mechanisms.
In view of this, particularly in cutting elements for drill bits, attempts have been made to improve the bond between the superhard facing table and the substrate by configuring the interface between the rear surface of the facing table and the front surface of the substrate so that these have inter-engaging projections and recesses to provide a non-planar interface. This may then provide a degree of mechanical interlocking between the facing table and substrate.
In cutting elements for drill bits, one part of the periphery of the element normally provides the cutting edge which is particularly subject to wear and impact loads in use. In some designs of cutting element, therefore, the configuration of the interface between the substrate and facing table has been made asymmetrical so as to increase the thickness and strength of the facing table in the region of the cutting edge and thereby increase the resistance of the cutting element to wear and impact.
Commonly, preform elements having a configured interface between the facing table and substrate have been manufactured individually. The usual method of manufacture has been to provide a solid substrate of the shape and size required for a single preform element and having a front face which is pre-formed with projections and recesses of the required shape. A layer of particulate diamond material is then applied to the front face of the substrate so that the diamond particles fill the recesses in the substrate and form a layer of the required thickness on the front surface of the substrate. The substrate with the applied diamond layer is then placed in the high pressure, high temperature press where the diamond particles are bonded together, with diamond-to-diamond bonding, to form the superhard layer, which in turn is bonded to the configured surface of the substrate.
It is found however that problems can arise with this manufacturing process where there is a non-planar interface between the superhard facing table and the substrate. For example, there may be difficulty in ensuring that all of the recesses on the front surface of the substrate are completely filled and packed with the diamond particles before the assembly is placed in the press. This is particularly the case with asymmetrical configurations of the interface. Also, the cost of manufacture of such preform elements is largely dictated by the number of handling steps involved in the manufacture and the size of the element has only a comparatively minor effect on the cost of manufacture, so that small preforms are relatively costly to manufacture.
The present invention sets out to provide an improved method of manufacturing preform elements where these disadvantages may be overcome.